Magnetic materials, including a variety of metals, alloys, and metal oxides are vital components in many technological applications. The physical properties of such materials are often affected by their magnetization resulting in such phenomena as magnetoresistance and magnetomechanical deformation. Magnetomechanical properties are observed when a change in magnetization causes strain within a solid. Magnetomechanical materials can be broadly classified into one of two types: (i) magnetoelastic materials in which the structural deformations are reversible once the magnetic field is removed; and, (ii) magnetoplastic materials in which the initial structure is not necessarily recovered on removal of the magnetic field. See, D. C. Jiles, “Recent advances and future directions in magnetic materials,” Acta Materialia, Vol. 51, No. 19, pp. 5907-5939 (Nov. 25, 2003). Shape memory alloys are a prime example of magnetoplastic materials, and deformations as large as 5-10% are often observed.
Magnetomechanical properties can be further defined by the type and origin of the deformation that is observed. Magnetostriction is defined as the fractional change in length of a material when magnetized. This phenomenon may occur spontaneously due to a magnetic phase transition, for example, having the temperature fall below the blocking temperature, or due to the application of an external magnetic field, and the resulting deformation is attributed to the rotation and movement of magnetic domains within the solid. Magnetostrictive solids, typically crystalline metals and alloys, have been extensively investigated for the past several years, with target applications in sensors and actuators.
Large magnetomechanical effects are also observed for a class of materials known as ferrogels, which are composites that include magnetic particles dispersed within a polymer gel matrix. The magnetodeformation of ferrogels differs from the customary magnetostriction of crystalline materials, in that ferrogels deform as a result of the interplay of pondermotive forces between magnetic particle and the elasticity of the polymer matrix. See Y. L. Raikher et al., “Magnetodeformational effect in ferrogel objects,” J. Magn. Magn. Mater., Vol. 258-259, pp. 477-479 (2003) and M. Zrinyi, et al., “Deformation of ferrogels induced by nonuniform magnetic fields,” J. Chem. Phys., Vol. 104, No. 21, pp. 8750-8756 (Jun. 1, 1996). Ferrogels may exhibit elongation, contraction, rotation, or bending in response to a magnetic field gradient, depending on specific structure of the ferrogel composite.
Lopatnikov et al. describe the deformation of a magnetically inert porous solid that has been infiltrated with a ferromagnetic fluid having a magnetomechanical effect in the presence of a magnetic field. See S. Lopatnikov et al., “A thermodynamically consistent formulation of magnetoporoeleasticity,” Int. J. Solids Structures, Vol. 35, Nos. 34-35, pp. 4637-4657 (December 1998).
Magnetically active materials that also contain defined pores or cavities have received little attention. Magnetomechanical properties have been observed in capsule-like structures with dimensions ranging from microns to tens of nanometers. For example, Lu et al. described polyelectrolyte “microcapsules” that contain magnetic Co@Au nanoparticles in the capsule wall. See Z. Lu et al., “Magnetic switch of permeability for polyelectrolyte microcapsules embedded with Co@Au nanoparticles,” Langmuir, Vol. 21, pp. 2042-2050 (2005). The application of an oscillating magnetic field disrupts the capsule wall, resulting in changes in the permeability of the capsule wall to macromolecules. Lu et al. proposed using the magnetically controlled permeability of these structures for drug delivery applications.
In another example, Shklyarevskiy et al. reported the deformation in a magnetic field of “nanocapsules” comprising a supramolecular assembly of sexithiophene amphiphiles. See I. O. Shklyarevskiy, et al., “Magnetic deformation of self-assembled sexithiophene spherical nanocapsules,” J. Am. Chem. Soc., Vol. 127, pp. 1112-1113 (2005). In that case, the deformation was ascribed to the large anisotropy in the diamagnetic susceptibility of sexithiophene-based molecule. Although capsule-like structures are potentially useful in dispersed forms, porous magnetic structures with macroscopic dimensions will also be desirable for membrane, sensor and actuator applications.
Much like the ferrogels discussed above, Makaki et al. recently described an example of such a porous magnetic structure, demonstrating that strongly bonded porous assemblies of ferrimagnetic metallic fibers (˜100 μm in diameter and 4 mm in length) undergo magnetomechanical actuation. See A. E. Markaki et al., “Magneto-mechanical actuation of bonded ferromagnetic fibre arrays,” Acta Materialia, Vol. 53, pp. 877-889 (2005). In the presence of an applied magnetic field the individual fibers tend to align with field resulting in the deformation of the fiber assembly. The authors report length changes of 0.2% for the fiber assembly, but did not investigate how the magnetomechanical distortion affected the size, shape, or connectivity of the pore structure.
Porous monolithic nanoarchitectures, or nanostructures, with compositions of such magnetic oxides as Fe3O4, γ-Fe2O3, and MnFe2O4 and the synthesis thereof were described by Long, et al. See J. W. Long et al., “Nanocrystalline iron oxide aerogels as mesoporous magnetic architectures,” J. Am. Chem. Soc., Vol. 126, pp. 16879-16889 (2004) and J. W. Long et al., “Synthesis and characterization of Mn—FeOx aerogels with magnetic properties,” J. Non-Cryst. Solids, Vol. 350, pp. 182-188 (2004), both of which are incorporated by reference herein in their entirety.
Also, mesoporous forms of SiO2, including nonmagnetic aerogels, have also recently been show to be effective vehicles for the delivery of common drugs, where drug release occurs passively over time, as controlled by the size, geometry, and connectivity of the pores, as well as the available surface area. See I. Smirnova et al., “Feasibility study of hydrophilic and hydrophobic silica aerogels as drug delivery systems,” J. Non-Cryst. Solids, Vol. 350, pp. 54-60 (Dec. 15, 2004) and J. Andersson et al., “Influences of material characteristics on ibuprofen drug loading and release profiles from ordered micro- and mesoporous silica matrices,” Chem. Mater., Vol. 16, No. 21, pp. 4160-4167 (Oct. 19, 2004).